The present embodiments relate to an arrangement for the transmission of magnetic resonance signals received with the aid of local coils.
Modern magnetic resonance units simultaneously receive a plurality of magnetic resonance (MR) signals via local coils that are positioned on patients. The local coils are part of a local coil array. The received magnetic resonance signals are preamplified, conducted from a central region of the magnetic resonance unit and fed to a shielded receiver, to be used at the shielded receiver for image processing.
Magnetic resonance signals have a high signal dynamic range, some of which covers more than 150 dBHz. In order to be able to process such signals without any perceptible deterioration of the signal-to-noise ratio, the components used to process the signal have an available dynamic range that is even more extended than the signal dynamic.
DE 101 48 442 C2 discloses a transmission method for magnetic resonance signals, the use of which results in greatly reduced dynamic requirements for analog/digital conversion of the receive path. The method is based on the fact that the receive signal amplitude is compressed by a compressor before the conversion. The signal amplitude is expanded again after the conversion. A linear transmission function of the overall system results.
The use of an analog amplitude compressor trades amplitude dynamic range for signal bandwidth. The compressed signal has a greatly expanded signal spectrum. In order to be able to achieve sufficiently exact signal expansion, the compressor output signal may be fed to the expander with as little distortion as possible to expand the output signal. Spectral clipping produces errors during expansion and therefore undesirable effects in the expanded output spectrum. This affects both harmonic components and intermodulation products.
The following problems therefore result. The filtering of harmonic components of the compressor output signal results in the formation of harmonic components in the expanded signal. If the expansion takes place in discrete time (e.g., digitally), the harmonic components appear due to backfolding in the first Nyquist band of sampling.
FIG. 1b shows an example of the Nyquist zones or bands 1 to 3, with the frequency in MHz shown on the x-axis, and the power level in decibel milliwatts shown on the y-axis.
Band limiting of the compressor output signal (e.g., filtering of intermodulation products) results in the formation of intermodulation products in the expanded signal.
These effects require the analog input bandwidth of the analog/digital converter or A/D converter (ADC) to be sufficiently large. If the sampling rate is not matched accordingly, this results in a significant reduction in the available dynamic due to interfolding of noise contributions from alias bands during sampling.
For reasons relating to structure or transmission (e.g., component availability, power requirement, bandwidth requirement for a radio transmission of the digital data), the sampling rate may not, however, be selected arbitrarily. If a radio transmission is inserted between compressor and ADC, the bandwidth requirement increases with the number of channels to be transmitted over practical limits.
In order to reduce the dynamic range to be processed by the ADC, a variable or switchable amplification may be inserted before the ADC. This has the disadvantage that amplification is preselected according to the maximum possible signal in each instance. If large signals are to be expected, amplification is reduced, and the quantization noise of the ADC moves to the foreground compared with the thermal noise from the patient.